I would suppose (and I'm no expert!) that no one man or woman suddenly crystallized the idea of hell as people envision it today. Nobody just started preaching it one day. Like a game of telephone (in a way), it began slowly, starting with highly educated philosopher/theologians like Augustine of Hippo. He wasn't the first Christian to claim hell was a real, physical place of punishment and suffering, but his writings had something like rock-star status. But even he didn't even think physical punishment was the primary punishment of hell.

His status as as a revered Church thinker gave the idea of hell real weight. By not long after he lived, people stopped talking about hell as a place that MIGHT exist as a place of eternal punishment. The assumption that hell DID exist became the predominate presumption. It wasn't long (a century or so) before it became unquestionable dogma, but the details built up quite slowly as the idea passed from person to person, leader to leader,

That probably surprises a lot of people who believe that hell was always part of Christianity, or that the so-called New Testament Christians believed and practiced much like Christians today.

In fact, the lack of hell is just ONE example of early Christian beliefs and practices that were starkly different from today's beliefs and practices.